The present invention relates to a cordierite honeycomb structure suitably used as a catalyst carrier for purifying automobile exhaust gas and a manufacturing method thereof.
Catalysts for purifying automobile exhaust gas are generally used in the form in which they are carried on a carrier. As a catalyst carrier of this kind has been utilized a honeycomb structure made of cordierite. In particular, emission gas control has become increasingly strict year by year, and thus, to keep up with the situation, i.e., to improve purification performance of the catalyst, a honeycomb structure with extremely thin partition wall as compared with conventional one has been proposed.
Thin-walled honeycomb structures, in which the wall thickness is 100 xcexcm or less and the open frontal area of the end surfaces are 83% or more in some cases, have a variety of favorable characteristics as a catalyst carrier; for example, (1) the catalyst can be activated in an early stage after engine start-up because of a low heat capacity and a high warming-up efficiency, and (2) a pressure loss during passage of exhaust gas through the catalyst carrier can be lowered due to a high open frontal area, and so utilization thereof can be expected in the future.
However, a thin-walled honeycomb structure presents a problem, due to the above described structural characteristics, of serious abrasion of the end surfaces of the frontal openings exposed to exhaust gas of high pressure discharged from the engine. In particular, a catalyst (i.e., catalyst carrier) has lately come to be placed in the immediate vicinity of an engine for the purpose of improving throughput, which becomes a factor in accelerating the abrasion of the end surfaces of the frontal openings.
As measures for solving such a problem have been studied a method of thickening only the partition walls in the vicinity of the end surfaces of the frontal openings of a honeycomb structure (JP-A-2000-51710), a method of applying and firing a glass component such as a glaze or water glass on the end surfaces of the frontal openings of the substrate of a honeycomb structure, and a method of similarly applying and firing a specific component of cordierite-forming raw materials (ibid.) and the like.
However, the method of thickening the proximities of the end surfaces of the frontal openings is not preferable because it lowers the open frontal area of the end surfaces of a honeycomb structure, although improving the abrasive resistance of the end surfaces of the frontal openings, thereby decreasing the merit of a small pressure loss of a thin-walled honeycomb structure. In addition, the method of applying and firing a glass component such as a glaze did not successfully provide sufficient abrasive resistance since the applied part was vitrified and became rather brittle, although making the part compact and increasing strength thereof.
Furthermore, the method of applying and firing a specific component of cordierite-forming raw materials (referring to raw materials, such as a mixture of talc, alumina and kaolin, which become cordierite by heat treatment such as firing) is not preferable because it causes the chemical composition of the applied part to deviate from the stoichiometrical composition of cordierite leading to a high coefficient of thermal expansion, thereby losing thermal shock resistance, although allowing compacting of the part to proceed. In addition, the method only made compact the proximities of the end surfaces of the frontal openings and was insufficient from the viewpoint of abrasive resistance as well.
The present invention was made in view of the problem of the prior art described above and it is an object of the invention to provide a honeycomb structure that does not decrease the merits of a thin-walled honeycomb structure, such as a small pressure loss, and is also excellent in the abrasive resistance of the end surfaces of the frontal openings, and manufacturing method thereof.
The inventors have carried out intensive studies, and found that the problem of the prior art described above can be solved by forming a reinforced part having been loaded with a cordierite powder at least on one end surface of the frontal openings of the substrate of a honeycomb structure made of cordierite thereby completing the invention.
In other words, the present invention provides a cordierite honeycomb structure characterized in that the structure has a reinforced part having been loaded with a cordierite powder at least on one end surface of the frontal openings of the substrate of a honeycomb structure made of cordierite.
A cordierite honeycomb structure of the present invention preferably has a reinforced part with which a cordierite powder has been loaded through a glass phase.
A cordierite honeycomb structure of the present invention preferably has reinforced parts only in portions of the end surfaces of the frontal openings of the substrate and of 10 mm or less from the end surfaces of the frontal openings of the substrate towards the cell-forming direction and it is effective when the thickness of the partition wall of the substrate is 100 xcexcm or less.
In addition, the present invention provides a manufacturing method of a cordierite honeycomb structure characterized in that the drying and/or firing are carried out after slurry containing a cordierite powder is applied to at least one end surface of the frontal openings of the fired or unfired substrate of a honeycomb structure, made of cordierite or a cordierite-forming raw material.
In a manufacturing method of the present invention, the drying and/or firing are preferably carried out after slurry containing a cordierite powder and colloidal ceramic is made to adhere to the structure and the drying and/or firing are preferably carried out after slurry containing a cordierite powder and glaze is made to adhere to the structure.
A cordierite honeycomb structure of the present invention has a reinforced part having been loaded with a cordierite powder at least on one end surface of the frontal openings of the substrate of a honeycomb structure made of cordierite.
The present invention provides a honeycomb structure that does not decrease the merits of a thin-walled honeycomb structure, such as a small pressure loss, and is excellent in the abrasive resistance of the end surfaces of the frontal openings.
A cordierite honeycomb structure of the present invention will be now described in detail.
A cordierite honeycomb structure of the present invention (hereinafter simply referred to as a xe2x80x9choneycomb structurexe2x80x9d) is characterized in that it has a reinforced part having been loaded with a cordierite powder. This reinforced part fills atmospheric pores of the wall surface of a cordierite honeycomb structure of which substrate is made of a cordierite powder and is fused through a firing step, and therefore it becomes compact and its porosity is lowered, compared with other parts. As a result, in addition to the improvement of the abrasive resistance of the part, the thermal shock resistance is not decreased because the reinforced part is loaded with a cordierite powder, which is also a material used for the substrate, and so the difference in coefficients of thermal expansion between the reinforced part and the substrate is small and the thermal shock resistance does not decrease.
A xe2x80x9ccordierite powderxe2x80x9d in this invention literally refers to a powder of cordierite (2MgO.2Al2O3.5SiO2), the chemical composition is 48 to 54% by weight of SiO2, 32 to 38% by weight of Al2O3 and 12 to 18% by weight of MgO, and its main crystalline phase is cordierite.
However, the effects of the present invention can not be attained, in the case that a material capable of being converted into cordierite by heat treatment after a xe2x80x9ccordierite-forming raw material,xe2x80x9d as is disclosed in JP-A-2000-51710, is adhered to a substrate and then the resultant is fired. This is because the resultant can not be called as one having been loaded with a xe2x80x9ccordierite powderxe2x80x9d. More specifically, the thickness of the partition walls of the adhered part is increased, which in turn only increases a pressure loss; as a result, no improvement in abrasive resistance is observed.
A xe2x80x9csubstratexe2x80x9d as used in the present invention refers to a honeycomb structure, a tubular structure having a large number of cells (penetrating pores) partitioned by a plurality of partition walls, and its whole shape, size, the number of cells and shape thereof, and the like are not particularly limited; however, the present invention is especially effective in a thin-walled honeycomb structure that exhibits a low abrasive resistance on the end surfaces of the frontal openings and has a partition wall thickness of 100 xcexcm or less.
A xe2x80x9creinforced partxe2x80x9d is possessed only on at least either one of the end surfaces of the frontal openings of the substrate and does not need to be possessed on both of the end surfaces of the frontal openings. Since only one end surface of the frontal openings is exposed to exhaust gas of high pressure placed towards the engine side, only the abrasive resistance of the part thereof needs to be improved. However, because of no need for consideration of the direction when placed, the reinforced part is preferably possessed on both end surfaces of the frontal openings.
The xe2x80x9creinforced partxe2x80x9d is compact and its porosity is low compared with other parts, and therefore widely spread formation thereof is not preferable because the amount of catalyst carried is decreased. Accordingly, the reinforced part is preferably possessed on a minimum portion for effecting enhancement in abrasive resistance, i.e., only the whole end surfaces of the frontal openings of a honeycomb structure and only the part within 10 mm from the end surfaces towards the cell-forming direction.
xe2x80x9cLoadingxe2x80x9d is used in the present invention refers to a state in which a cordierite powder is bound to the substrate in any form, and cordierite after having been loaded does not need to be a powder. For example, when a cordierite powder is loaded to the substrate by heat treatment, the part of a cordierite powder, depending on the temperature, sometimes becomes like glass to be fused to the substrate through a glass phase, or is completely fused to be integrated into the substrate. These states are all included in the xe2x80x9cloadingxe2x80x9d in the present invention.
While a substrate that is manufactured by extrusion molding is generally used, when slurry is made to adhere thereto, the substrate may be made of cordierite or made of a cordierite-forming raw material, or the substrate may be fired one (hereinafter referred to as a xe2x80x9cfired bodyxe2x80x9d) or unfired one (one only dried, not fired yet after extrusion molding; hereinafter referred to as a xe2x80x9cdried bodyxe2x80x9d). However, firing after loading is essential for a body made of a cordierite-forming raw material and a dried body.
A method of loading a cordierite powder is not particularly limited. The methods, for example, include a spray method, a coating method with a brush, and a method coating with a sponge or the like impregnated with a slurry. More particularly, a method in which a substrate is immersed in slurry containing a cordierite powder and after the slurry is applied thereto, the substrate is dried and/or fired (so called the dipping method) is preferable. This is because the dipping method is simple and can provide a homogeneous carried state.
When loading is conducted by the dipping method, it is preferable to prepare slurry using a cordierite powder with an average particle size of 10 xcexcm or less as measured by the laser method, in order to stabilize the slurry to attain homogeneous loading.
Water, for example, can be used as a dispersing medium for slurry and the slurry concentration is not particularly limited. However, the concentration of solid matter is preferably about 10 to 60% by weight in order to prepare homogeneous slurry with few precipitates.
To the above described slurry may be, as necessary, added an additive such as a surfactant used to prepare normal slurry.
Furthermore, an additive for strengthening the binding between colloidal ceramic, glaze or the like, a cordierite powder and a substrate is preferably added.
xe2x80x9cColloidal ceramicxe2x80x9d in this invention refers to a colloidal ceramic particle with an average particle size of 0.2 xcexcm or less. Addition of this particle together with a cordierite powder to slurry is preferable because it permits homogeneous loading of a cordierite powder and drying or heat treatment such as firing evaporates the moisture of the colloidal matter to create a strong adhesive effect.
This method can improve the abrasive resistance, if the substrate is a fired body, only even by drying the substrate at a relatively low temperature of about 100 to 200xc2x0 C. without firing after slurry is made to adhere.
Moreover, if the firing is carried out after increasing the temperature to about 1400xc2x0 C., a temperature of the proximity of the melting point (1450xc2x0 C.) of cordierite, part of a cordierite powder becomes like glass and is fused to and is strongly bound to the substrate through a glass phase; therefore, it is preferable from the viewpoint of further improving the abrasive resistance.
Although the kind of ceramic for the colloidal ceramic is not particularly restricted, colloidal silica and colloidal alumina, which are commercially available, are preferably used. The solid matter ratio of cordierite powder to colloidal ceramic is preferably from about 5:95 to 20:80 by weight, to avoid the effect of a large extent of contraction when the colloidal ceramic is crystallized or vitrified in the process of heat treatment.
A xe2x80x9cglazexe2x80x9d in this invention refers to a formulation that forms glass of silicates after firing.
When this is added to slurry along with a cordierite powder, homogeneous loading of a cordierite powder becomes possible as in the case of colloidal ceramic. In addition, the agent lowers the melting point of a cordierite and thus the resultant is fused to the substrate relatively easily, or permits a cordierite powder to be melted and to be integrated into the substrate, thereby simply and easily leading to a strongly bound state.
Further, it is to be noted that a glaze, different from colloidal ceramic, is highly effective in adhesion only after it is fired and vitrified. That is, the effects of a glaze described above cannot be acquired simply by drying without firing.
Constituents constituting a glaze is not particularly restricted and a feldspar-based glaze with a vitrification temperature of 1250xc2x0 C. or above is preferably used so as not to extremely lower the melting point of the substrate part to which the glaze adheres, and so a cordierite-based glaze in which about 30 to 40% by weight of cordierite is formulated in advance is further preferable in order to decrease the coefficient of thermal expansion of the glaze.
The cordierite-based glaze mentioned above can be prepared, for example, by wet grinding and mixing a cordierite powder to be carried in the substrate together with glaze raw materials of feldspar, silica, clay, etc. using a trommel.
In this case, a cordierite powder in the glaze is the same material as, but much finer than, a cordierite powder to be carried in the substrate, thereby constituting part of the vitrified constituent.
For the ratio between cordierite powder and glaze, as the amount of the glass constituent produced from the glaze is increased, the structure is liable to be brittle, thereby lowering the abrasive resistance. Accordingly, in a cordierite-based glaze, the ratio of cordierite powder to glaze is preferably about 1 to 1.
The substrate to which slurry is applied as mentioned above can form a reinforced part by drying and/or firing.
xe2x80x9cDryingxe2x80x9d here refers to an operation in which a dispersing medium in the slurry is removed at a relatively low temperature of about 100 to 200xc2x0 C. and a cordierite powder and the like in the slurry are bound to the substrate. On the other hand, xe2x80x9cfiringxe2x80x9d refers to an operation(s) in which a cordierite powder and the like in the slurry are sintered, fused, and melted and integrated into the substrate by heat treatment at a relatively high temperature of about 500 to 1400xc2x0 C.